The present invention relates generally to magnetic field sensing devices and more specifically to peak magnetic field sensing devices which include non volatile storage of the peak magnetic field strength.
There are many situations where there is a need to measure and hold the value of a peak magnetic field which occurred during a given time period. One example is the need to measure and record or store the maximum electrical current flowing through a conductor during a given time interval. The current to be measured may also consist of only a current pulse of very short duration. The need includes the requirement that this measuring and holding of the current value be accomplished with a minimal change in the current, minimal change in the wiring and minimal change in the transmission characteristics or impedance of the conductor.
In the past, one approach to this need has been to cut the conductor and insert a resistor so that the current to be measured passes through the resistor causing a resulting voltage drop. In some instances the insertion of the resistor can be accomplished at a circuit connector. The voltage across the resistor is then measured, for example with a voltmeter, and since the voltage drop is proportional to the current in the conductor the current can then be determined. This approach is invasive in that it physically changes the circuit and the impedance of the current path. In addition this approach requires that there be space available for the resistor insertion and voltmeter arrangement. In actual practice, the need for current measurement in systems frequently exists when there is simply not sufficient space available for the resistor and voltmeter in the system.
Another approach to measuring and holding the value of a current involves placing an inductive pickup consisting of one loop or of many loops around the current carrying conductor and measuring the induced voltage in this pickup. By integrating this induced voltage over time, the current in the conductor can be determined.
The resistor insertion approach and the inductive pickup approach have shortcomings. The resistor insertion approach requires that a voltage be measured. The inductive pickup approach requires that the voltage be measured and stored as a function of time. These requirements become more difficult when for example the voltage occurs as a short pulse. Also, in many practical applications the data storage equipment is necessarily at some distance from the current measurement point.
Magnetic sensors are another prior approach to current measurement. Hall effect sensors for example can use a Hall element constructed from a thin sheet of conductive material with output connections perpendicular to the direction of current flow through the element. When subjected to a magnetic field normal to the element, as provided by the current to be measured in a nearby conductor, the element responds with an output voltage proportional to the magnetic field strength. The voltage output is very small and requires additional electronics, either on or off chip, to achieve useful voltage levels as well as to provide the excitation current through the element and storage of the acquired information.
Magnetoresistive sensors may also be used for current measurement. Materials such as permalloy can be given a preferred magnetic orientation by application of an external field, which may be supplied by a nearby current strap. If a magnetic field is then applied perpendicular to the current and in the plane of the permalloy thin film material, the direction of magnetization will rotate toward the direction of the magnetic field. The angle through which the magnetization direction rotates depends on the amplitude of the external field produced by the current to be measured. The resistance of the permalloy element decreases as the direction of magnetization rotates away from the direction in which the current in the magnetoresistive element flows, and this resistance change is representative of the magnetic field due to the current to be measured. Magnetoresistive sensors require electrical power to provide the current in the sensor and will require signal conditioning and readout electronics.
Also, fluxgate magnetometers as well as other types of magnetometers have been used to measure currents. However all of these approaches require separate signal conditioning and storage circuitry.
Thus a need exists for simple non-invasive current sensing and storage capability.